1. Field of the Invention
The invention is related to test instruments for electronic components, and more particularly, to test instruments for measuring noise parameters of an electronic component.
2. Description of the Related Art
For many purposes, it is desirable to have test equipment capable of accurately measuring the noise parameters of a device under test (DUT). The accuracy of such noise measurements is particularly important for components intended to operate at microwave frequencies, since lower-frequency components usually can meet noise specifications more easily. The smaller the uncertainty of the test instrument in measuring the noise parameters of a product, the better will be the manufacturer's worst-case noise specifications for the product.
The noise parameters of a component are usually specified in terms of noise factor F or equivalent noise temperature T.sub.e. The noise factor of a device is defined as the ratio of the signal-to-noise ratio available from the device output to the signal-to-noise ratio delivered to the input of the device, at a standard reference temperature of T.sub.Ref =290.degree. K. Noise factor is often expressed in dB, at which time it is sometimes referred to as a noise figure.
A well-known class of methods for measuring F involves applying two different noise signals with respective noise temperatures T.sub.C and T.sub.H to the input of the device, and measuring the noise power of the output in each case. For the input noise temperature of T.sub.C, the measured output noise power is referred to herein as N.sub.1. For the input noise temperature of T.sub.H, the measured output noise power is referred to herein as N.sub.2. The noise factor can be calculated from these numbers using the well-known formula, ##EQU1## where EQU Y.sub.m .ident.N.sub.2 /N.sub.1
and ENR is the "excess noise ratio" specified for the noise source when hot. ENR is given by EQU ENR.ident.(T.sub.H /T.sub.Ref -1). (eq. 1)
Other parameters may also be determined once N.sub.1 and N.sub.2 are known. For example, the equivalent noise temperature T.sub.e of the device is given by EQU T.sub.e =(T.sub.H -Y.sub.m T.sub.c)/(Y.sub.m -1),
the gain G of the device can be derived from the formula: EQU G=(N.sub.2 -N.sub.1)/kB (T.sub.H -T.sub.C),
(where B is the bandwidth of the DUT or the test equipment, whichever is narrower, and k is Boltzmann's constant), the noise power N.sub.a added by the device is given by: EQU N.sub.a =N.sub.1 -GkT.sub.C B,
and the noise figure in dB of the device is given by: EQU F(dB)=10.sub.10 F.
In a practical noise parameter measurement system, the noise of the test equipment must be taken into account when measuring the noise of a DUT. A typical measurement model is shown in FIG. 1, in which a noise source 12 is coupled to the input port of a DUT 10, the output of which is coupled to the input port of a receiver 14. The noise source 12 is capable of supplying the two different noise temperatures, T.sub.H and T.sub.C. In this setup, it is well known that the noise factor F.sub.DUT for the DUT can be calculated from the cascade formula: ##EQU2## where F.sub.rcvr is the noise factor of the receiver, F.sub.sys is the noise factor of the entire system, and G.sub.DUT is the gain of the DUT. F.sub.sys can be determined by any of several known methods, including the 2-point technique described above. F.sub.rcvr may be determined using the same method, but with the DUT bypassed such that the noise source feeds directly into the receiver. Typically F.sub.rcvr is determined during a calibration step and stored in memory. G.sub.DUT can be calculated from the formula, ##EQU3## where
N.sub.2sys is the output noise power of the entire system, with the input noise at T=T.sub.Hsys,
N.sub.1sys is the output noise power of the entire system, with the input noise at T=T.sub.Csys,
N.sub.2rcvr is the output noise power of the receiver only, with input noise at T=T.sub.Hrcvr, and
N.sub.1rcvr is the output noise power of the receiver only, with the input noise set at T=T.sub.Crcvr.
A simplification is possible if the noise source hot and cold temperatures T.sub.H and T.sub.C are assumed not to vary between the system noise measurements and the receiver noise measurements.
The cold noise temperature supplied to the system may be the noise temperature with the source turned off or discontented, and the hot noise temperature may be that applied when the noise source is turned on or connected. Noise sources are generally specified by the manufacturer in terms of their ENR at the hot temperature and T.sub.H can be calculated from equation (eq. 1) above. T.sub.C is usually assumed to be equivalent to the ambient temperature. The values of T.sub.C and either T.sub.H or ENR, possibly at various frequencies, are then typically entered into the system manually.
In the past, noise module measurement instruments were stand-alone and frequency-limited. They required additional frequency conversion hardware to perform noise measurements over a wide band. In addition, they could not measure S-parameters of the DUT, so that if a user wanted to measure both the S-parameters of a DUT and the noise parameters, two separate instruments were required.
S-parameters are typically measured using a vector network analyzer (VNA). Two examples of VNAs are the HP8510 manufactured by Hewlett-Packard Company, and the Wiltron 360, manufactured by Wiltron Company. A VNA is described in detail in U.S. patent application Ser. No. 07/176,202, filed Mar. 31, 1988 entitled "Microwave Measurement System and Associated Method," by Bradley, Grace, Thornton Finch, incorporated by reference herein.
The above formulas for calculating the noise parameters of a DUT all assume a perfect impedance match between the noise source and the DUT input port and between the DUT output port and the receiver input port. In the past, noise parameter measurement instruments have used the above formulas or their equivalents, and therefore required users either to use a DUT which has a perfect 50 ohm input and output impedance, or to insert isolators, circulators or carefully chosen attenuation pads at the input and output terminals of the DUT. The restriction to DUTs with perfect impedance inputs and outputs clearly limited the usefulness of the test instrument, but the insertion of isolators, circulators, or pads was cumbersome and often added its own noise. Additionally, the loss of any of these impedance correction components would directly increase the noise figure of the receiver and therefore add to the uncertainty for that reason alone. Isolators and circulators are less lossy than pads, but each is useful only within a limited frequency range. The impedance mismatch of the DUT is known to be one of the major sources of measurement uncertainty in prior-art noise module measurement systems. The issue is especially important at microwave frequencies, where mismatches are more common.